This invention relates to a power output circuit.
A prior-art protective circuit for a power output circuit known from, for example, Japanese patent application laid open specification No. 48-3064 protects the load (loudspeaker) of the power output circuit of the OCL (Output Condenser Less) type.
In order to prevent the loudspeaker from being damaged by a deviation in the output d.c. level (mid-point potential) of the OCL type power output circuit, the known protective circuit detects that the output d.c. level has exceeded a predetermined voltage, and using the detection output it turns "off" switching means such as a relay disposed between the output of the power output circuit and the loudspeaker. Thus, the voice coil of the loudspeaker can be protected from burning attributed to the passage of direct current.
On the other hand, the inventors' study has revealed that, in case of constructing this sort of protective circuit into a semiconductor integrated circuit, detecting transistors for effecting the above detection are destroyed by any abnormal operation of the power output circuit or various surge voltages applied to the semiconductor integrated circuit.
When the detecting transistors have been destroyed, there is the danger that the turning-off of the switching means such as relay disposed between the output of the electric circuit such as OCL type power output circuit and the loudspeaker load will not be effected, resulting in the burning of the loudspeaker load.
The inventors' study has also revealed the following. For protecting the load such as loudspeaker from the abnormal output of the electric circuit such as OCL type power output circuit, it is favorable from the standpoint of enhancing the reliability of the protective operation that, not only the output d.c. level as stated above, but also the abnormal current increase of output transistors attributed to thermal runaway, short-circuit of the load, or the like is detected and that the switching means is turned "off" on the basis of either of these detection outputs. Detecting transistors for detecting the current of the output transistors are destroyed in the same manner as described above.